A Syrian government airstrike on an opposition-held district in Aleppo has killed at least two people in what was alleged to have been a chlorine gas attack.

The attack on the city's eastern Zabadieh neighbourhood saw at least four barrel bombs dropped on the area, one of which purportedly released the chlorine gas.

A father recounted dramatic moments of gasping for breath and cowering with his family on the top floors of their apartment building as a choking gas filled the hallway.

It came hours after the Russian military, which is fighting alongside Syrian government forces in the civil war, promised a daily, three-hour cease-fire for Aleppo to allow humanitarian aid in to besieged areas.

Lt Gen Sergei Rudskoi of the Russian military's General Staff said the cease-fires will be observed from 10am-1pm local time starting on Thursday, in order to facilitate the distribution of aid.

The Organisation for the Prohibition of Chemical Weapons said that reports of possible chemical weapons use in Syria "are of great concern".

The Netherlands-based agency said in a statement that the use of chemical weapons by anyone under any circumstances is "reprehensible", adding it continues to examine any credible reports it received.

Khaled Harah, a first responder in the rebel-held part of Aleppo, said a government helicopter dropped four barrel bombs on the neighbourhood of Zabadieh and that one of them released chlorine gas, leading to the deaths of a mother and her two children.

The report, which was posted online on Thursday, could not be independently verified and it was not clear how it was determined that chlorine gas was released.

The Britain-based Syrian Observatory for Human Rights, an activist group that tracks the civil war in Syria, also reported that government barrel bombs struck the neighbourhood. It had reports of two killed and several people suffering breathing difficulties.

The Observatory made no mention of chlorine gas.

Abdelkafi al-Hamdu, a resident of Aleppo, said he saw two airstrikes from his in-law's balcony, about 30 metres away.

He said the first blast released a gas he identified by the smell as chlorine, but the wind was blowing in the other direction, lessening the odor.

He took cover in the apartment but began experiencing severe difficulties breathing, so he took his wife and daughter with him and tried to leave the building. But the odor grew stronger as they descended the stairs, so they returned to the higher floors to wait out the effects.

Accusations involving use of chlorine and other poisonous gases are not uncommon in Syria's civil war, and both sides have denied using them while blaming the other for using it as a weapon of war.

Last week, the Syrian government and the opposition traded accusations of using chlorine, also in Aleppo.

Chlorine gas is a crude weapon, fatal in high concentrations while in lower doses, it can damage lungs or cause severe breathing difficulties and other symptoms such as vomiting and nausea.

AP

Meanwhile, activists say at least 20 civilians have been killed in airstrikes on Raqqa, the de facto capital to the Islamic State group in northern Syria.

The local activist group Raqqa is Being Slaughtered Silently says the strikes also cut the city's water supply.

The group said a security building and a water pumping station were hit in the city, among other targets on Thursday morning.

The Britain-based Syrian Observatory for Human Rights also reported the airstrikes, saying 24 civilians were killed, along with six others whose affiliation and identities could not yet be confirmed.

Both groups said the strikes were launched by Russian jets, though it was not clear how they determined that.